The invention has been developed in connection with the manufacture of barrels for downhole pumps, such as are used in oil wells. Thus, because of this history, the invention will be described in connection with pump barrels, and the problems associated therewith. However, it is to be understood that the invention will also extend to the manufacture of long, thin-walled, small inside diameter tubes utilized in other applications.
By way of background, a downhole pump consists essentially of a steel barrel containing a reciprocating plunger, for moving the fluid, and suitable valve means, for controlling the admittance and discharge of fluid into and out of the barrel. The barrel is long, typically having a length in the range 6-30 feet. It is of small internal diameter, typically falling in the range 1 1/4"-3 1/4". And it is thin-walled, typically having a tube wall thickness in the range 1/8"-1/4". In its simplest form, the barrel comprises a straight tube having strict tolerances with respect to internal diameter and concentricity of the internal diameter relative to the longitudinal axis of the barrel. Since efficient pumping requires minimal clearances between the plunger and the barrel inner surface, internal wear of the barrel wall is the prime determinant of barrel life, particularly, as so often happens, when abrasive particles are present in the pump fluids.
Initially, these barrels were constructed from soft metal pipe, but wear on the barrel rapidly resulted in loss of tolerance limits. In order to increase the service life of the barrels and, to provide abrasion resistance therefor, it was found necessary to harden the internal wear surface thereof.
Historically, the earliest hardened barrels were produced utilizing conventional heat treatment techniques. The barrel would be through-heated to the austenitic temperature in a furnace and then quenched by dropping it into a body of water. However, whilst the barrels so formed exhibited increased wear resistance, these through hardened barrels lacked the requisite toughness or impact strength properties, and, additionally, were found to be undesirably prone to stress corrosion cracking.
Attempts were then made to harden the inner surface only, utilizing an internal flame followed by a quench ring. The problem with this process resided in the inability to limit the heating to the inside layer of the thin wall, and thus prevent austenite being generated deeper in the wall than was desirable. Stated otherwise, the system did not permit enough heat to be applied in a short enough time and the quench coolant to be applied soon enough thereafter in sufficient volume, so that only a thin internal surface layer was heated to the austenitic temperature and that layer was cooled sufficiently quickly to form a sufficiently hard martensitic microstructure.
It has also become known to harden carburized or carbonitrided tubing by induction heating of the outside diameter of the barrel, or by conventional furnace heating allowed by a water quenching step. The inner surface on-y of the starting tube would have a high carbon content. Thus hardening would occur only at the inner surface section, while the thick outer section would remain tough and ductile , as is desired. The major disadvantages of this technique are associated with the lengthiness and high cost of the carburizing stage. Also, because of the high cost of carburizing, there is a tendency to shorten the carburizing treatment. This results in a very thin case being produced During the subsequent honing operation, this case is frequently found to have been completely, or partially, removed.
A recent development in the barrel hardening field employs steel tubing having sufficient carbon content to harden without provision of selective surface enrichment with either carbon or nitrogen. This process utilizes a low frequency induction heating, typically 3kHz to 10kHz, to through-heat the entire barrel wall from the outside inwardly. The tube, upon attaining the requisite quenching temperature, is quenched on the inside only, to thereby form an internal hardened case. In some cases, an external quench may also be applied. In this process, a short coil is positioned on the outside wall of the tube. The coil is functional to traverse the barrel, heating limited areas at a time, to thus limit the power requirements. The problem with this process is that the outer section of the barrel wall becomes hardened and is in tension. This condition is particularly susceptible to hydrogen embrittlement and cracking.
A search of the prior art has been conducted. Several patents, exemplary of which are U.S. Pat. Nos. 2,556,236 to Strickland, and U.S. Pat. No. 2,547,053 to Somes et al, were located which disclosed internal electromagnetic induction heating, followed by quenching, to thereby harden the internal bores of short workpieces, such as cylinders, bushings or the like.
The prior art patents specified above disclosed broadly the metallurgical concepts of:
rapidly heating a rotating, translating, tubular workpiece by means of an internally positioned, closely conforming magnetic induction coil, to thereby raise the temperature of a thin surface layer to a value greater than the austenitizing temperature, without heating the relatively thick outer portion of the wall to the austenitizing temperature; and PA1 immediately quenching the austenitic surface layer to produce a tube having a relatively thin, hardened, martensitic inner surface layer, or case, having a Rockwell C hardness (HRC) in the order of 58-61 and exhibiting a compressive residual stress pattern, said case being contained by a relatively thick outer wall section which is in a tough, substantially non-hardened condition. PA1 the provision of coaxial tubular power conductors extending along the axis of the tube from its first end and being connected with an induction coil for heating, said conductors being separated by insulation and otherwise being contiguous, said inner conductor forming a bore for supplying cooling water to the hollow coil; PA1 the provision of a quench ring or head, secured to the end of the coil and connected to a water supply mandrel extending along the axis of the tube from its second end, said quench head having a spray outlet directed toward the tube s second end, so that the cooling water discharges away from the coil; PA1 the bore of the coil communicating with an outlet so that the coil cooling water is discharged toward the second end of the tube; PA1 said workpiece, coil, quench head, conductors and quench water mandrel being vertically disposed. PA1 to supply high frequency, high density power rapidly to the workpiece, to obtain quench-hardening temperature of the thin surface layer and ensure non-hardening of the outer portion of the wall; and PA1 immediately quenching to thereby successfully harden the thin surface layer. PA1 transmitting an adequate quantum of radio frequency power to the coil, given the long length of the conductor and thus the attendant line losses; PA1 developing adequate electromagnetic coupling between the internally disposed induction coil and the workpiece, to thereby enable the necessarily large amount of power to be drawn from the generator; PA1 developing high surface power density in the workpiece layer to be hardened, thereby supplying sufficient heat at a sufficiently rapid rate to the layer to attain a temperature therein substantially exceeding the austenitizing temperature and to thus effect the necessary carbon dissolution and homogenization in the metal before significant through wall heat transfer can result; PA1 preventing back-flow of the quench water into the induction heating zone, which back-flow would counteract the heating effect; PA1 delivering a sufficient volume of quench water at a suitable temperature to the heated austenitic surface layer, instantly upon termination of heating, to produce a case having martensitic microstructure; PA1 overcoming the mechanical limitations imposed by the small internal diameter of the tube and the warpage problem associated With the thin-walled nature of the long tube; PA1 and performing the above on the commercially available grades of tubing with normal manufacturing dimensional tolerances. PA1 delivering sufficient radio frequency AC power to an induction coil disposed in the bore of a long, thin-walled, small-diameter, ferrous tube, and coupling the coil to the tube to electromagnetically induce a high power density in a surface layer of the tube, so as to raise the temperature of a longitudinal portion of the layer to a value substantially greater than the austentizing temperature; PA1 said tube having a length greater than about 8 feet and an inside diameter in the range of 1 1/4 to 3 1/4 inches, said tube preferably having a length in the range 8 to 32 feet, an inside diameter in the range 1 1/4 to 2 1/2 inches, and a wall thickness in the range 1/8 to 1/4 inch; PA1 one of said coil and tube rotating and translating relative to the other, whereby the entire length of the layer is progressively heated to said temperature; delivering liquid coolant to the fully heated portion of the layer through an internally placed quench ring, as soon as said portion reaches said temperature; said power being supplied at a sufficient rate for a short enough period of time and said coolant being supplied at a sufficient rate soon enough after heating, so that the outer segment of the tube wall, beyond the layer, is left unhardened and the inner surface layer, preferably having a depth less than about 1 mm, is converted to a martensitic case, preferably having a surface hardness Rockwell Hardness C (HRC) value greater than about 58 and an effective case thickness to HRC 50 greater than about 0.5 mm; and propelling substantially all of said coolant, after it has quenched, through the already hardened end of said tube, so that it does not back into the heating zone. PA1 (1) delivering at least 115 KW of 180-400 kilohertz AC power to the coil through a tubular, electrically conductive power lead, said coil also being connected with a tubular, electrically conductive ground lead, said leads being substantially coaxial and spaced apart to form an annular passage, said coil being formed of electrically conductive tubing forming a bore which communicates with the power lead bore and the annular passage, the last turn of the coil being perforated to form an inductively active quench ring, said coil being closely coupled to the workpiece so as to deliver at least about 22 MW/m.sup.2 power density (which corresponds with the output of the 115 KW in a 1 1/2" I.D. tube); PA1 (2) delivering sufficient coolant through the inner conductor bore and the annular passage to the quench ring whereby the coolant is applied to the fully heated portion of the tube layer almost instantly upon termination of heating and quenchs the layer to produce a martensitic case; and PA1 (3) mechanically propelling the coolant, after it has quenched the heated section of the tube layer, out through the hardened end of the tube.
Mechanically, these prior art patents disclosed the following assemblies:
The prior art exemplified by these patents further recognized that it was necessary:
However, it is to be noted, that the above-mentioned patents employed short tubes having large internal diameters, typically about 6.5 inches. Additionally, the composition of the workpieces comprised alloy steels, which steels are relatively easily hardenable, entailing less criticality in the hardening process thereof with regard to the heating and quenching parameters.
There are, however, serious problems which arise when one attempts to apply these prior art techniques to elongated, thin-walled, small diameter tubes of a plain carbon steel or non-alloy containing feedstock, such as those used to form downhole pump barrels. These problems have presumably heretofore prevented the application of the internal induction heating technology, in a commercially viable process, to such barrels.
More particularly, the problems to be addressed included:
The ideal characteristics for a pump barrel comprise a tube formed of a composite material having a tough and ductile microstructure in the bulk of the wall thickness and a hardened internal surface layer or case. These properties impart abrasion resistance to the inner wear surface and impact resistance to the outer surface, together with a minimization in propensity to stress corrosion cracking. Preferably, the hardened case should have a thickness of less than 1.0 mm, a hardness exceeding HRC 58, and a substantially uniform martensitic microstructure. Furthermore, the barrel should exhibit a favorable residual stress distribution, with the inside surface preferably being in the highly compressive condition, and the outer core being non-hardened and exhibiting a low tensile residual stress condition. Typically, a desirable hardness profile would demonstrate a sharp demarcation between the hardened and non-heat affected zones.
The ideal characteristics for a manufacturing process to produce such a pump barrel would include relatively high treating speed and low cost.
It is an objective of the present invention to provide a solution to the afore-mentioned problems and further to provide a long, thin-walled, small-diameter tube having the preferred physical and metallurgical characteristics for a pump barrel.